Project Summary Our lab has recently identified a family of 15 novel secreted plasma proteins, termed C1q/TNF-related proteins 1-15 (CTRP1-15), and we have found that many play prominent roles in regulating glucose and lipid metabolism (7-23). CTRP15, also termed myonectin, is a skeletal muscle-derived hormone and is the focus of this proposed project. Previous studies implicate myonectin as a postprandial hormone that facilitates lipid uptake from circulation into peripheral tissues (7). Preliminary experiments utilizing a whole-body knockout (KO) mouse model suggest that male KO mice challenged with a high fat diet (HFD) exhibit deficiencies in serum lipid clearance as well as differences in lipid deposition among liver and adipose tissues relative to their wildtype (WT) littermates. These data provide physiological evidence that myonectin is an important factor in regulating lipid metabolism to achieve whole-body energy homeostasis. The goal of the proposed project is to use HFD-fed KO mice as a model to study myonectin?s tissue-specific functions and mechanisms of action in vivo. Understanding how myonectin mediates tissue crosstalk to control metabolism will likely provide insights into how disruption of the metabolic network can lead to diseased states and perhaps reveal novel strategies to treat or prevent obesity-related metabolic disorders. The first specific aim is to elucidate the tissue-specific mechanisms underlying the impaired lipid clearance phenotype in HFD-fed KO mice. Radiolabeled lipids will be used to track acute lipid uptake and utilization in KO and WT mice to assess postprandial lipid partitioning. The contribution of alterations in lipoprotein lipase (LPL) activity to the high serum triglyceride levels in KO mice will be determined by measuring enzyme activity in different tissues derived from HFD-fed WT and KO mice. Finally, the ability of myonectin to directly modulate LPL activity will be analyzed by measuring enzyme activity in tissue explants derived from WT mice and treated with recombinant myonectin. The second specific aim is to determine, in a tissue-specific manner, the lipid metabolism processes that myonectin regulates. Fatty acid oxidation, de novo lipogenesis, and triglyceride synthesis will be evaluated in primary adipocytes and hepatocytes isolated from HFD-fed WT and KO mice to determine if alterations in any of these pathways contribute to the observed difference in lipid deposition. Additionally, primary cells will be isolated from WT mice and treated with recombinant myonectin before subjecting them to the same assays to determine if myonectin can directly affect these processes. Together, results from these studies will provide mechanistic insight at the tissue level underlying the impaired ability of KO mice to clear postprandial lipids and the difference in lipid partitioning. Additionally, these studies will reveal whether the observed phenotypes reflect the lack of direct myonectin action or are secondary to other alterations caused by myonectin deficiency.